If they see competition, they program their sons to load up on pheromones.

In the wild, mice are social and promiscuous, with males competing to attract females. Under typical lab breeding programs, mice are paired and mate without the competition, taking away the adaptive advantage of being able to attract a lot of ladies.

University of Utah scientists have been studying what happens genetically as a population of wild-derived mice adapts to captive, non-competitive conditions over generations. They've discovered that the female mice have some control over the sexiness of their future sons.

Their results support a longstanding evolutionary concept called the "sexy son hypothesis" which suggests that when male attractiveness comes at a cost of reduced fitness elsewhere—think peacocks and their ridiculous tails, which make them slow but really get the peahens' feathers ruffled—it still pays off for the females to mate with them because their own sons will inherit the attractive trait and get more of their own ladies, keeping more of their genes in the game.

To see this evolutionary strategy play out in the lab, the scientists compared the offspring of two populations of mice: one adapted over several generations to traditional captive conditions, known as "monogamous-line," and another adapted to living in social conditions where they had to compete for territory and mates, known as "promiscuous-line." All the offspring were then raised in individual cages so that social influences during their own lives didn't influence their biology.

When the Utah scientists subjected the mice to fitness tests, they found that the promiscuous line males lived up to their name, siring more offspring, although at the cost of reduced survivorship. They found no difference between the females.

The difference they found between the promiscuous-line and monogamous-line males was in their expression of some pheromones in the mice's urine (called major urinary proteins, or MUPs) that attract females. The increased pheromone production is inherited not through the DNA itself, but through the chemical modification of the DNA, which can block transcription of nearby genes. This process, known as DNA methylation, leads to what's termed epigenetic inheritance—inheritance of traits that aren't directly coded by the DNA sequence itself.

The social experience of the mouse mothers appears to control the methyl modification near the gene for a pheromone—in sexually competitive environments, the methylation is reduced, and the male offspring produce sexy smelling urine (sexy to other mice, at least). The scientists also found that male mice responded to the same social environments by passing on the opposite: epigenetic changes that led to a down-regulation of the same pheromone, although the effect was much smaller than the effect exerted by the mothers. Perhaps, the researchers hypothesize, it's a male attempt to reduce the mate competition they will eventually face from their sons?

The much stronger effect is that the moms try to enhance their sons' sex appeal if it's going to give them an advantage in a sexually competitive environment. Unfortunately, that sex appeal came at a cost of decreased survival. The researchers write that "inheritance of up-regulated MUPs is likely adaptive because females have odor preferences for male scent marks with higher MUP concentration. These results represent one of only a few cases where parental social experience adaptively modifies progeny phenotype."

The interesting question is if similar behavior occurs in animals that produce fewer offspring with higher requirements for maturation - primates and humans being the top examples.

Ultimately the job of a mother is to ensure the passing on of her genetic material. For mice which have large litters, and can have many the winning strategy would be to get pregnant as often as possible, and to favor traits which will enable her offspring to find mates easily.

However, for animals that have single offspring at a time, and which require longer periods before offspring are mature, the strategy would seem different, especially for mothers which are saddled with the bulk of the burden of care. For these it would seem like the ideal traits would be those that promote a long lifespan, and show willing and capability to provide for offspring.

I'm sure somewhere out there is work on providing a game theoretic model of reproduction that would include these cases.

The interesting question is if similar behavior occurs in animals that produce fewer offspring with higher requirements for maturation - primates and humans being the top examples.

Ultimately the job of a mother is to ensure the passing on of her genetic material. For mice which have large litters, and can have many the winning strategy would be to get pregnant as often as possible, and to favor traits which will enable her offspring to find mates easily.

However, for animals that have single offspring at a time, and which require longer periods before offspring are mature, the strategy would seem different, especially for mothers which are saddled with the bulk of the burden of care. For these it would seem like the ideal traits would be those that promote a long lifespan, and show willing and capability to provide for offspring.

I'm sure somewhere out there is work on providing a game theoretic model of reproduction that would include these cases.

Even so, while not so relevant in modern times, historically I think the same model would work for humans. I don't know that it _does_, but I think it makes evolutionary sense.

In a wealthy with good resource access there are plenty of females with which to breed, no resource bottlenecks, and a genetic advantage to siring as many offspring as possible at once. If you're sufficiently competitive reproductively then your gene lines will spread and become more common.

On the other hand, in a scarce society then both females (because of gestation) and resources are bottlenecks. In that event there's less advantage to having a bunch of women carrying your offspring at once (since you'd end up with "downtime), and an advantage in spreading the resource investment over a longer period of time (instead of "paying for" 5 kids a year, you have the same total at the rate of 1 per year). Even if the male doesn't specifically pay for their offspring, the economics seem to me to be equivalent at the social level.

So, in a resource-rich environment you want a lot of kids, fast (genetically, at least). In a resource-starved environment you both get more benefit from reproducing more slowly, and are bottlenecked by the availability of reproductive women anyway.

The interesting question is if similar behavior occurs in animals that produce fewer offspring with higher requirements for maturation - primates and humans being the top examples.

Ultimately the job of a mother is to ensure the passing on of her genetic material. For mice which have large litters, and can have many the winning strategy would be to get pregnant as often as possible, and to favor traits which will enable her offspring to find mates easily.

However, for animals that have single offspring at a time, and which require longer periods before offspring are mature, the strategy would seem different, especially for mothers which are saddled with the bulk of the burden of care. For these it would seem like the ideal traits would be those that promote a long lifespan, and show willing and capability to provide for offspring.

I'm sure somewhere out there is work on providing a game theoretic model of reproduction that would include these cases.

The interesting question is if similar behavior occurs in animals that produce fewer offspring with higher requirements for maturation - primates and humans being the top examples.

Ultimately the job of a mother is to ensure the passing on of her genetic material. For mice which have large litters, and can have many the winning strategy would be to get pregnant as often as possible, and to favor traits which will enable her offspring to find mates easily.

However, for animals that have single offspring at a time, and which require longer periods before offspring are mature, the strategy would seem different, especially for mothers which are saddled with the bulk of the burden of care. For these it would seem like the ideal traits would be those that promote a long lifespan, and show willing and capability to provide for offspring.

I'm sure somewhere out there is work on providing a game theoretic model of reproduction that would include these cases.

Even so, while not so relevant in modern times, historically I think the same model would work for humans. I don't know that it _does_, but I think it makes evolutionary sense.

In a wealthy with good resource access there are plenty of females with which to breed, no resource bottlenecks, and a genetic advantage to siring as many offspring as possible at once. If you're sufficiently competitive reproductively then your gene lines will spread and become more common.

On the other hand, in a scarce society then both females (because of gestation) and resources are bottlenecks. In that event there's less advantage to having a bunch of women carrying your offspring at once (since you'd end up with "downtime), and an advantage in spreading the resource investment over a longer period of time (instead of "paying for" 5 kids a year, you have the same total at the rate of 1 per year). Even if the male doesn't specifically pay for their offspring, the economics seem to me to be equivalent at the social level.

So, in a resource-rich environment you want a lot of kids, fast (genetically, at least). In a resource-starved environment you both get more benefit from reproducing more slowly, and are bottlenecked by the availability of reproductive women anyway.

Except that this is the opposite of what actually happens. Organisms that, for whatever reason, lack the resources through their own effort to move the needle on their offspring's survival tend to have lots of offspring (think fish laying eggs). Those that exert a significant influence have few or one offspring at a time (think great apes).

At a high level, humans seem to be very adaptable in this regard... some economic circumstances lead (on average) to large human families and other economic circumstances lead (on average) to very small ones. Humans also infected their reproduction cycle with self-aware intelligence, so individual-level outcomes have loads of confounding factors.

Currently, the humans who have the most children are down-and-out. It's been shown that the poor have more children than the rich, the fat have more children than the fit, the uneducated have fewer children than the well educated.

I guess that means the evolutionary advantage for a human male goes to the ones who don't have any standards at all, and will knock up anybody. If you're willing to have kids with the women other guys wouldn't go near, then there will be more like you in future generations.

If our lives are like a movie, it will not be "Star Trek", we went to see "Idiocracy". It shows in public policy too. The same year we stopped funding the ITER experiment we had an all time high percentage of the population on food stamps.

The scientists also found that male mice responded to the same social environments by passing on the opposite: epigenetic changes that led to a down-regulation of the same pheromone, although the effect was much smaller than the effect exerted by the mothers. Perhaps, the researchers hypothesize, it's a male attempt to reduce the mate competition they will eventually face from their sons?

This makes no sense whatsoever: of what benefit is it to the genes to be passed on for one generation and then be less likely to survive beyond that? A trait by which a father which leaves children less likely to reproduce will quickly be supplanted by genes of successful mice spawning more successful mice, who can then go on to spawn yet another successful generation.

What seems far more likely is that this is a check on the negative effects of enhanced promiscuity.Were each generation to just increase the markers unchecked, the downsides of lower survivability might exceed the benefits of greater promiscuity. Resetting the levels back toward 'normal' (though by slightly less than the female's increase thus resulting in a new 'normal' being slightly more attractive) improves the longevity of the subsequent males. If conditions are such that competition is no longer so high, the female can then decide whether or not the promiscuous/survival tradeoff is still needed instead of the trait being a permanent one-way ratchet toward poor health.

My postulate allows for generational adaptation, the postulate that a male does not want his offspring to be as successful as he was does not lead to long-term success.

Kate Prengaman / Kate is a science and environmental reporter living in Yakima, Washington. She writes about everything from emerging energy technology to persistent environmental problems and she really likes plants.